Windows Defender in Windows 8.1 Will Not Turn On

I recently updated Windows 8 to Windows 8.1.  Windows Defender had been on in Windows 8 and even after the update to 8.1.  I turned it off momentarily but have been unable to turn it back on.  To my knowledge, I have no other anti-virus programs running.  How can I turn it back on?

cf. http://answers.microsoft.com/en-us/protect/forum/protect_defender-protect_start/cant-get-windows-defender-to-start-on-my-windows-8/1f4d269a-b153-4b4b-bebb-8ae90df1051b

Hi, I'm having the same issue, Windows Defender wont turn on on my Windows 8 Pavilion laptop.  Originally I had Norton Internet Security running till it expired, now I can't get the Windows Defender to start via the Action Center.  I tried to start it by following your steps regarding the Service window but received this error message: The Windows Defender Service service on local computer started and then stopped.  Some services stop automatically if they are not in use by other services or programs...

I recently updated Windows 8 to Windows 8.1.  Windows Defender had been on in Windows 8 and even after the update to 8.1.  I turned it off momentarily but have been unable to turn it back on.  To my knowledge, I have no other anti-virus programs running.  How can I turn it back on?

Hi, I'm having the same problem, that is, turning on Windows Defender.  I followed both procedures on your post but was unable to activate defender.  The following two messages occur:

1.  "This app has been turned off and isn't monitoring your computer.  If you're using another app to check for malicious or unwanted software, use Action Center to check that app's status."

2.  "Windows could not start the Windows Defender Service service on Local Computer.  Error 577: Windows cannot verify the digital signature for this file. A recent hardware or software change might have installed a file that is signed incorrectly or damaged, or that might be malicious software from an unknown source."

The second message including Error 577 occurs when I attempt to run defender as administrator.

Any suggestions?  Thanks for your help!

The Windows Defender was on my computer, Windows 8.1 but then Norton came with a good offer and I took it for one year. That has now expired and they expect me to pay the full price so I tried to get the Defender to work but an empty file appears. I contacted Microsoft, a guy looked into my computer and told me that the Defender has been uninstalled and that it is impossible to install it without losing all the programs that I have installed which would be a huge bother. What can I do? There must be a simple way of reinstalling the Defender? And how can it have uninstalled itself?